The Influence of ENSO on Northern Midlatitude Ozone during the Winter to Spring Transition

The influence of El Nino-Southern Oscillation (ENSO) on northern midlatitude ozone during the period January-March (JFM) is investigated using various observations and a chemistry-climate model. The analysis reveals that, during El Nino events, there are noticeable anomalously high total ozone column (TOC) values over the North Pacific, the southern United States, northeastern Africa, and East Asia but anomalously low values in central Europe and over the North Atlantic. La Nina events have almost the opposite effects on TOC anomalies. The longitudinal dependence of midlatitude ozone anomalies associated with ENSO events during the period JFM is found to be related to planetary waves. Planetary waves excited by tropical convection propagate into the middle latitudes and give rise to longwave trains (Pacific-North American pattern) and shortwave trains along the North African-Asian jet. These wave trains affect ozone in the upper troposphere and lower stratosphere (UTLS) by modulating the midlatitude tropopause height and cause TOC anomalies by changing the vertical distributions of ozone. In addition, synoptic-scale Rossby wave breaking increases on the poleward flanks of the enhanced westerly jet during El Nino events, leading to a stronger eddy-driven meridional circulation in the UTLS and hence causing TOC increases over the North Pacific, the southern United States, northeastern Africa, and East Asia and vice versa for La Nina events. It is also found that the contribution of changes in Brewer-Dobson circulation due to anomalous planetary wave dissipation in the stratosphere during ENSO events to TOC changes in the middle latitudes for the period JFM is small, not more than 1 Dobson unit (DU) per month.